The smaller clubs still play in the ERJFA, the better clubs would perhaps join forces with the better NRJFA clubs in a combined top division, the winners go to the HL. something like that is fairly straightforward and very workable. I'm fairly sure that the bigger Dundee area clubs will easily cope with the demands of the HL and attract renewed interest, and after a few years there may be more than one or two in there anyway, along with the likes of Banks O'Dee.
Let's remember they already have a Tayside/North Cup in the Juniors, so both leagues know the lie of the land.